1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling a transmission in an unlicensed band and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system is developed by 3rd Generation Partnership Project (3GPP) to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs). A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system increases peak data rate and throughput, and includes advanced techniques, such as carrier aggregation (CA), licensed-assisted access (LAA) using LTE, etc.
Network operators propose to offload network traffic of the LTE/LTE-A system to an unlicensed band, to ease load of the network traffic. For example, the eNB provides services to a UE via the unlicensed band. However, the UE is not able to receive a HARQ feedback at a time instant in the unlicensed band, because resources in the unlicensed band are not available at the time instant for the eNB to transmit the HARQ feedback on a carrier of the unlicensed band due to that the carrier is busy or occupied. That is, the operation of the UE in the unlicensed band may not be performed regularly due to the uncertainty of the resources in the unlicensed band. Correspondingly, the eNB cannot receive a new transmission or a retransmission at a time instant corresponding to the HARQ feedback in the unlicensed band because the UE is not able to transmit the new transmission or the retransmission on a carrier of the unlicensed band at the time instant due to that the carrier is busy or occupied, and the operation of the eNB is also affected.
Thus, how to solve the transmission/reception of the HARQ feedback and the corresponding (re)transmission in the unlicensed band is an important problem to be solved.